Digital Subscriber Line (DSL) technology, including e.g. ADSL, ADSL2, (S)HDSL, VDSL, VDSL2 up to the upcoming G.fast, during all its history, attempted to increase the bit rate in the aim to deliver more broadband services to the customer. Unfortunately, copper loops deployed from a Central Office (CO) to customer premises (CPE) are rather long and do not allow transmission of data with bit rates more than few Mb/s. Therefore, to increase the customer available bit rates, modern access networks use street cabinets, MDU-cabinets, and similar arrangements, also referred to as distribution points (DP): the cabinet or other DP is connected to the CO by a high-speed fiber communication line, e.g., gigabit passive optical network (GPON) and installed close to the customer premises. From these cabinets, high-speed DSL systems, such as Very-High-Bit-Rate DSL (VDSL), provide connection to the CPE. The currently deployed VDSL systems (ITU-T Recommendation G.993.2) have range of about 1 km, providing bit rates in the range of tens of Mb/s. To increase the bit rate of VDSL systems deployed from the cabinet, the recent ITU-T Recommendation G.993.5 defined vectored transmission that allows increasing upstream and downstream bit rates up to 100 Mb/s. Vectoring will also be used in upcoming G.fast.
One important component or stage of DSL system is initialization (or training). During the initialization, lines that join to the vectored group provide the ability for existing active lines to accommodate crosstalk from new lines, provide the ability for joining lines to accommodate crosstalk from active lines, and finally provides joining lines with proper PSD and channel setting and bit loading.
This application addresses for example issues in initialization of vectored lines that use high and very high frequencies.
One of the elements conventionally used to perform initialization is a special operations channel (SOC) that is established between a DP modem and a CPE modem during initialization to communicate messages between modems that are necessary to support the initialization procedure and convey special training signal and sequences to train the transceivers. The SOC is usually built in a very robust way and operates during initialization only. Examples of SOC that are used in current DSLs are described for example in ITU-T Recommendations G.993.2, G.993.5.
One serious issue with vectored lines is high crosstalk, especially when very high frequencies (such as 100 MHz and higher) are used. During initialization and training, when FEXT (far-end crosstalk) between lines is not cancelled, signals transmitted over lines are “visible” in all other lines. One issue with that is the initialization and training signals and messages transmitted over one line are received in another line(s), thus producing inaccurate or even false (or ghost) training, which may increase the training time or even harm the active lines operating in the vectored group and serving customers. In emerging new generation of DSL technology, such as G.fast, FEXT from adjacent line may be stronger than the direct signal which may ghost initialization and cross-line training not only possible, but even probable. One of typical issues is that neighboring SOC signals disturb the direct channel estimation process, especially training of FEQ and some other procedures that require operation in the presence of high crosstalk.